1. Field of the Invention
The present invention relates to subterranean drilling operations, and more particularly, to drilling fluids demonstrating improved shale inhibition and improved environmental performance, and methods of using such drilling fluids in drilling operations in subterranean formations.
2. Description of the Prior Art
A drilling fluid used in connection with drilling a well in a subterranean formation is any number of fluids (gaseous or liquid) and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill well bores into subterranean formations. Drilling fluids are used to, inter alia, cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drill cuttings from the well bore.
During drilling of subterranean well bores, it is not uncommon to encounter strata comprising reactive shales. As referred to herein, the term “shale” will be understood to mean materials such as certain types of clays (for example, bentonite) and related subterranean materials that may “swell,” or increase in volume, when exposed to water. Reactive shales may be problematic during drilling operations because of, inter alia, their tendency to degrade when exposed to aqueous media such as aqueous-based drilling fluids. This degradation, of which swelling is one example, can result in undesirable drilling conditions and undesirable interference with the drilling fluid. For instance, the degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until such time as the cuttings can be removed by solids control equipment located at the surface. Degradation of drilled cuttings prior to their removal at the surface greatly prolongs drilling time, because shale particles traveling up the well bore break up into smaller and smaller particles, which increasingly exposes new surface area of the shale to the drilling fluid, which leads to still further absorption of water, and further degradation.
Shale disintegration may also adversely impact “equivalent circulation density” (“ECD”). ECD is affected by the solids content of the drilling fluid, which increases if surface solids control equipment cannot remove shale from the drilling fluid. Plastic viscosity (an indicator of size and quantity of solids) is an important parameter in determining drilling rate. Maintenance of appropriate ECD is extremely important in situations where a well is being drilled wherein a narrow tolerance exists between the weight of the drilling fluid needed to control the formation pressure, and the weight of the drilling fluid that will fracture the formation. In such circumstances, minimizing shale degradation provides improved control of the density of the drilling fluid, and enhances the probability of successfully drilling a well bore.
Shale degradation may substantially decrease the stability of the well bore, which may cause irregularities in the diameter of the well bore, e.g., the diameter of some portions of the well bore may be either smaller or greater than desired. In an extreme case, shale degradation may decrease the stability of the well bore to such extent that the well bore will collapse. Degradation of the shale may also, inter alia, interrupt circulation of the drilling fluid, cause greater friction between the drill string and the well bore, or cause the drill string to become stuck in the well bore. Accordingly, the complications associated with shale swelling during drilling may greatly increase the cost of drilling.
A traditional method of inhibiting shale swelling during drilling to attempt to minimize such complications has been to use an oil-based drilling fluid as opposed to an aqueous-based drilling fluid. However, oil-based drilling fluids are often environmentally undesirable because they may be toxic to marine plants and animals. Accordingly, environmental regulations enacted by numerous countries have curtailed the use of oil-based drilling fluids. Consequently, water-based drilling fluids are preferred because they likely have a more benign effect on the environment than oil-based drilling fluids. However, the use of aqueous drilling fluids results in added complications, due to interactions between components of the subterranean formation such as reactive shales and the aqueous fluids.
To counteract the propensity of aqueous drilling fluids to interact with reactive shales in the formation, a shale inhibiting component may be added to the aqueous drilling fluid. As referred to herein, the term “shale inhibiting component” will be understood to mean a compound that demonstrates a propensity for inhibiting the tendency of a sample of shale to absorb water. Amphoteric materials are one type of water-based shale inhibitor that have been used in the past. Amphoteric materials are believed to function, inter alia, by attaching to the shale substrate, thus preventing water ingress. Amphoteric inhibitors are environmentally undesirable, however, especially in heavily regulated areas, because they demonstrate low biodegradability and high toxicity. Potassium chloride is another material that has been utilized as a shale inhibitor. Although potassium chloride is widely used as a shale inhibitor in the North Sea, it is considered to be only moderately effective at inhibiting the swelling of shale. Furthermore, potassium chloride is environmentally unacceptable in other areas of the world. For example, potassium chloride is unsuitable for use in regions such as the Gulf of Mexico, because its concentration of potassium ions is troublesome for certain types of marine life, e.g., shrimp. Potassium chloride is also disfavored in certain regions, such as the Middle East, where wells are drilled in close proximity to aquifers due to concerns that the potassium chloride will contaminate the aquifer. Polyglycols have also been used as shale inhibitors in water-based drilling fluids, but have not reached satisfactory inhibition levels. Partially hydrolyzed polyacrylamides (“PHPA”) have also been utilized in many regions, but these have been found to cause formation damage, and are environmentally undesirable.